In photographic silver halide emulsion layers gelatin remains the principal binder. The demand for rapid processing, dimensional stability, and image sharpness has led to a steady decrease in the ratio of gelatin to silver halide. This ratio expressed by weight is further called "gesi". For the calculation of such ratio values the amount of silver halide is expressed therein as its equivalent amount of silver nitrate.
Thin emulsion layers resulting from lower amounts of gelatin as e.g. for multilayer color reversal films with a thickness of about 10 .mu.m and for rapid access medical X-ray films with a thickness of at most 5 .mu.m have been described in SPSE Handbook of Photographic Science and Engineering--Woodlief Thomas, Jr. Editor, SPSE--A Wiley Inter-science Publication--John Wiley & Sons, New York (1973), p. 514).
From "Photographic Chemistry" edited by Pierre Glafkides, Vol. I, (1958), p. 314-315, it is known that a silver halide emulsion containing low amounts of gelatin with respect to silver halide will develop more rapidly, and that the gamma infinity will be reached more quickly.
Experimentally it has been found that in that way a higher sensitivity, also called "speed", can be attained for quite a lot of materials as well as an increase in covering power, defined as density measured per gram of developed silver in the region of maximum density.
However a decrease in the amount of gelatin and a reduction of the coating thickness brings about a series of problems of which coating unevenness and coating streaks are the most important. In terms of sensitometry the appearance of an enhanced fog and pressure sensitization or desensitization phenomena are the most well-known.
Coating a layer containing gelatin requires a viscosity of the coating liquid which is of particular importance and which is decisive for the choice of the coating technique. Modern coating apparatus used for high speed coating of aqueous coating liquids containing gelatin on web type film or paper supports are the slide-hopper coater and the curtain coater, described e.g. in U.S. Pat. Nos. 3,632,374 and 3,867,901 and in U.S. Pat. Nos. 2,761,791 and 4,113,903 and in EP-A 0 382 058 respectively.
When applying a method operating at high coating speed it is of utmost importance that after coating the layer containing gelatin on cooling solidifies as rapidly as possible and quickly reaches a degree allowing the drying to remove water from the coated layer, with a vigorous dry air current without distorting or destructing the already solidified layer containing gelatin in gel form. Only by combining rapid coating speed with short solidifying and drying stages the production speed of recording materials based on coatings from layers containing gelatin can be increased substantially.
It is known that silver halide emulsion coating solutions with low viscosities are more prone to instability in the coating bead before contact with the web, which causes coating defects, and undesired interlayer mixing in multilayer coating. Various thickening agents have been proposed to facilitate the coating of dilute photographic emulsions as has been described e.g. in U.S. Pat. No. 3,767,410. Many of these agents are not entirely compatible and exhibit problems such as the production of haze, brittleness, etc. Moreover use of anionic macromolecular polyelectrolytes is restricted to coatings in a particular pH region because of undesired coagulation of the gelatin, to a certain extent, in their presence.
As a practical matter e.g. in order to manufacture gelatinous photographic silver halide emulsion layers coated from low amounts of gelatin, it is therefore preferred to dispense with thickening agents and still to obtain a desired viscosity in the coating liquids containing gelatin correlated with a proper shear rate for fault-free coating. Nowadays coating velocities are increased by application of slide-hopper and curtain coating techniques, allowing also more rapid drying after coating as use is made of more concentrated coating compositions. Said more concentrated compositions require a higher viscosity, in particular when low amounts of gelatin are present in the said coating composition.
Coating of silver halide emulsion layers having a low gelatin content is particularly important when materials are envisaged that after image-wise exposure are subjected to rapid processing within a time of less than 90 seconds and even more if ultra rapid processing is applied as e.g. processing within a total time cycle from 20 to 60 seconds during which development, fixing, rinsing and drying of the photographic material is completed.
In this case even making use of gelatin having an increased viscosity is insufficient as has e.g. been described in EP-A 0 532 094. The preparation of gelatin suitable for producing aqueous solutions having high viscosity and rapid solidification is described e.g. in EP-A 0 025 494. Another method to provide a gelatin having a viscosity which is high enough in order to be useful in coating compositions of hydrophilic layers is a so-called "chain extended gelatin" as has been disclosed in WO 92/09008, wherein the viscosity is increased without gel formation or formation of insolubles. However if lower concentrations of gelatin are present in coating compositions it is not always possible to enhance the viscosity to the desired level as has already been suggested hereinbefore. Moreover the use of thickening agents, more in particular of anionic macromolecular polyelectrolytes, in order to obtain gelatinous solutions having strong shear rate dependency, is in many cases disadvantageous for rapid solidification of the layer or layers coated therefrom as sticking may form an additional problem.
Moreover, application of ultra-rapid processing normally proceeds at an elevated temperature e.g. in the temperature range of 30.degree. C. to 45.degree. C. in an automatic roller transport apparatus. Under such conditions silver halide emulsion layers having a low gelatin content have too low an abrasion resistance, may show roller marks and give rise to sludge formation.
Besides light-sensitive silver halide photographic materials also (photo)thermographic recording materials, wherein layers whether or not comprising silver salt in the absence of gelatin are present, demand for rapid coating and drying capability.